1. Field of the Invention
The present invention relates to a hydrodynamic bearing device, a spindle motor using the same, and a disc recording and reproduction apparatus including the same.
2. Description of the Related Art
The disc recording and reproducing apparatus may be, for example, a hard disc drive (HDD), or a DVD recorder, which magnetically or optically reads/writes data from/to a disc type recording medium such as a magnetic disc or an optical disc (hereinafter, referred to as a disc) while rotating the disc.
Disc recording and reproducing apparatuses are required to have more capacity and a higher speed of data transfer. Particularly, more HDDs are used in AV equipment and mobile equipment in recent years. For use in such equipment, a high durability is also required in addition to the above requirements. Therefore, it is desired that rotation of the discs becomes more rapid and more stable with a high precision. Furthermore, the reliability on such a rotation has to be maintained high for a long period of time.
Hydrodynamic bearing devices are suitable for such rotary drive systems having a high speed, high precision and high durability.
FIG. 12 is a cross sectional view showing an example of a conventional hydrodynamic bearing device. The hydrodynamic bearing device is mounted on an HDD.
An outer surface of a sleeve 31 is fixed to a base 35. Into the sleeve 31, a shaft 32 is inserted so as to be rotatable around its central axis. An upper end of the shaft 32 is fixed to a central portion of a hub 36 by, for example, press fitting. A flange 33 is fixed to a lower end of the shaft 32 in close vicinity to a step portion 31C provided in a lower opening of the sleeve 31. The lower opening of the sleeve 31 is sealed with a thrust plate 34.
On an inner surface of the sleeve 31, radial dynamic pressure generating grooves 31A and 31B are provided (see portions indicated by broken lines in FIG. 12). On an upper surface and a lower surface of the flange 33, thrust dynamic pressure generating grooves 33A and 33B are provided. The radial dynamic pressure generating grooves 31A and 31B and the thrust dynamic pressure generating grooves 33A and 33B may be, for example, grooves having a herringbone pattern. Oil 42 fills most of gaps between the sleeve 31, the shaft 32, the flange 33 and the thrust plate 34. Particularly, the oil 42 covers the entire surface of the radial dynamic pressure generating grooves 31A and 31B and the thrust dynamic pressure generating grooves 33A and 33B.
To an outer surface of the hub 36, magnetic discs 39 can be fixed coaxially with the shaft 32. In general, a plurality of magnetic discs 39 may be attached. A spacer 40 is provided between inner peripheral portions of the magnetic discs 39. A clamper 41 is fixed to an upper portion of the hub 36 with, for example, a screw 43 to press the inner peripheral portions of the magnetic discs 39 downward. In this way, the magnetic discs 39 are fixed to the hub 36.
On an inner surface of the hub 36, a magnet 38 is provided. Opposite to the magnet 38, a stator 37 is provided on the base 35. The stator 37 and the magnet 38 form a driving force generating section for disc rotation.
The above-described hydrodynamic bearing device operates as follows.
When a current flows through the stator 37, a magnetic field is generated in a core portion of the stator 37. The magnetic field generated between the stator 37 and the magnet 38 applies a rotational force to the hub 36. Thus, the shaft 32, the hub 36, and the magnetic discs 39 integrally rotate having the shaft 32 as an axis.
As they rotate, the oil 42 flows along the radial dynamic pressure generating grooves 31A and 31B and gathers in the vicinity of inflection points of the dynamic pressure generating groves. As a result, the pressure of the oil 42 is raised in a gap near the central portions. This high pressure due to a pumping action maintains a gap between the sleeve 31 and the shaft 32 stable. Thus, a rotational axis of each of the magnetic discs 39 does not substantially move in a radial direction of the sleeve 31.
Similarly, the oil 42 flows along the thrust dynamic pressure generating grooves 33A and 33B and gather to their central portions. As a result, pressure of the oil 42 is raised in gaps above and below the flange 33. This high pressure due to a pumping action maintains a gap between an upper surface of the flange 33 and the sleeve 31, and a gap between a lower surface of the flange 33 and the thrust plate 34 stable. Thus, a rotational axis of each of the magnetic discs 39 does not substantially tilt from an axial direction of the sleeve 31. Furthermore, the shaft 32 is not substantially displaced in the axial direction.
In this way, the above-described hydrodynamic bearing device rotates the magnetic discs 39 rapidly with a high precision and a stable manner.
In the above-described hydrodynamic bearing device, a gap between an outer peripheral surface 33C of the flange 33 and a surface of the step portion 31C of the sleeve 31 is uniform across the entirety of the outer peripheral surface 33C, and is sufficiently large compared to the space between the upper surface of the flange 33 and the sleeve 31, and the gap between the lower surface of the flange 33 and the thrust plate 34. Thus, friction due to the oil 42 on the outer peripheral surface 33C of the flange 33 is small. Accordingly, so-called torque loss is small. This leads to high rotation efficiency of the magnetic discs 39 and low power consumption.
Another type of conventional hydrodynamic bearing device is also known (see for example, Japanese Patent Gazette No. 2966725). Unlike the above-described hydrodynamic bearing device, the gap between the outer surface of the flange and the inner surface of the sleeve varies in the axial direction as shown in FIG. 13. In this type of hydrodynamic bearing device, a protrusion 33D is provided on the outer peripheral surface 33C of the flange 33. Thus, the gap between the outer peripheral surface 33C and a surface of the step portion 31C of the sleeve 31 is narrow in the vicinity of the tip of the protrusion 33D compared to that in gaps U and D above and below the tip.
In general, a sealing force of the oil 42 becomes strong as a width of the space to be filled becomes small. Thus, the sealing force of the oil 42 is stronger in the gap near the tip of the protrusion 33D than in the gaps U and D above and below the tip. Therefore, it is difficult for the oil 42 to move beyond the protrusion 33D. This means that the oil 42 cannot easily circulate from above the flange 33 to below the flange 33, and vice versa.
When the shaft 32 is tilted from the axial direction of the sleeve 31 or displaced in the axial direction, both the gap between the upper surface of the flange 33 and the sleeve 31, and the gap between the lower surface of the flange 33 and the thrust plate 34 change. This causes a gap in the pressure of the oil 42 above and below the flange 33. However, since the oil 42 cannot easily move beyond the protrusion 33D, the pressure gap cannot be eased by circulation of the oil 42. The pressure gap brings back the shaft 32 to its original position. In this way, an excessive tilt and an excessive displacement in the axial direction of the shaft 32 can be both prevented effectively.
In some of the conventional hydrodynamic bearing devices, flanges are provided to shafts as described above. An outer peripheral surface of the flange has a distance from the central axis of the shaft (i.e., diameter) larger those of a side surface of the shaft and other surfaces of the flange. Thus, a large portion of torque loss is due to friction between the outer peripheral surface of the flange and a lubricant. Therefore, it is effective to keep the friction due to a lubricant on the outer peripheral surface of the flange low for keeping the total torque loss low. Keeping the torque loss low is preferable because it results in keeping the rotation efficiency of discs high and accelerating power saving.
In the conventional hydrodynamic bearing devices as shown in FIG. 13, the stability of the shaft is improved as described above. However, the torque loss increases. Thus, it is not preferable in view of power saving.
Furthermore, since the lubricant cannot be easily circulate around the flange, circulation of the lubricant along the thrust dynamic pressure generating grooves is not accelerated. Thus, it is difficult to reduce a period of time for the pressure of the lubricant to be raised sufficiently in the gaps above and below the flange after the flange started to rotate. Specifically, it is difficult to reduce a period of time necessary for achieving stable rotation of the discs after startup of the spindle motor. Particularly, in HDDs, it is difficult to further speed up the startup from a halting state or standby state.
An object of the present invention is to provide a hydrodynamic bearing device which accelerates circulation of a lubricant in the vicinity of a flange while keeping a torque loss low, thereby achieving both power saving and reduction of a period of time necessary for stabilizing rotation and improving a durability of the motor in startup and halting.